I always saw Celestia as the one that went "Destroy. Everything" simply because she controls the most powerful element there is; The sun. I doubt that she'd be able to keep a leash on her powers. In my mind; just by fully releasing her powers she does damage to the environment, and fights using brute force to overcome her opponent (because she has so much of it).

Luna, on the other hand, is the one with control in my book. Lots of power, but no as much as Celestia, and knows how to use it just so, that she gains maximum efficiency from her spell and doesn't waste any magic, nor damage the environment or anyone save her target.